1. Field of the Invention
The present invention relates to a block noise removal device for removing block noise that is produced when decoding information data produced as a result of performing compression-coding the information data on each of a plurality of blocks.
2. Description of the Related Art
Compression-coding, which adopts the MPEG (Moving Picture Expert Group) system, is implemented in order to reduce an amount of information when transmitting and/or recording video or speech signals or the like. In such MPEG encoding processing, compression of the information amount is effected by performing quantization processing after performing conversion to produce a DCT coefficient for each frequency region. The DCT coefficient is by performing a Discrete Cosine Transformation (called ‘DCT’ hereinbelow) for each two-dimensional unit block of the video signal. The larger the quantization step used in the quantization processing, the larger the compression rate. However, quantization noise is produced as a result of some values being discarded. Block noise is a typical example of such quantization noise. In other words, because various processing is performed on each two-dimensional unit block as mentioned above in the MPEG encoding processing, the boundary of the block become evidence when the two-dimensional unit blocks are decoded. For the video signal, compression of a brightness signal or color difference signal is the norm but various signal formats such as an RGB signal may be considered.
Therefore, a method for detecting such block noise in a video signal that has undergone MPEG decoding and removing the block noise has been proposed. One example of such methods is disclosed in Japanese Patent Application Kokai (Laid Open) No. 2000-50275. With this block noise removal method, the difference between adjacent pixels in a horizontal direction is first obtained for a video signal that has undergone MPEG decoding. When the differential value is greater than a predetermined threshold value, it is determined that the corresponding point is the block boundary (See FIGS. 3A to 3C of Japanese Patent Application Kokai No. 2000-50275). By performing filtering processing on the block boundary portion, a block noise reduction is implemented by smoothing a sharp level change between adjacent blocks which constitutes the origin of the block noise.
With respect to a video signal that represents a picture such that the level of the video signal increases (or decreases) gradually in a horizontal direction within each block as shown in FIG. 1 of the accompanying drawings, the difference between adjacent pixels is always a fixed value other than zero. Therefore, there has been the risk that this difference will be erroneously judged as being the block boundary BB.
Because the block boundary in a video signal whose resolution has been resized or in a video signal that has undergone analog conversion fades after MPEG decoding, the block boundary cannot be accurately detected.